It is desirable to provide coherent reception of transmitted signals in a communications system, for which it is necessary to estimate parameters (amplitude, phase, frequency offset, and delay) of the communications channel which affect signal synchronization. A wireless CDMA system typically has multiple paths with multi-path fading, so that such parameters continuously change and must be estimated in an ongoing manner. Accordingly, accurate channel estimation in a CDMA system presents a substantial challenge.
In current CDMA systems, it has been proposed to allocate four channels for each user for synchronization and data communication. These channels are referred to as the pilot channel (P), for synchronization purposes; the fundamental channel (F), for voice signals and low-rate data transmission; the supplemental channel (S), for high-rate data communication, and the control channel (C), for very low-rate data communication for control purposes. One or more of the last three channels, i.e. the data channels, need not be used by a particular user at any time. For simplicity the following description refers primarily to the fundamental channel, but it should be understood that the same comments apply for any one or more of the data channels.
For efficient operation of the CDMA system, it is desirable for the transmit signal power allocated to the pilot channel (i.e. the relative gain of the pilot channel) to be small relative to that of the data channels. The pilot channel can be used for channel estimation, but its relatively low power can result in poor phase accuracy and poor amplitude tracking, so that channel estimation accuracy is not sufficient. For example, the much stronger fundamental channel constitutes interference when estimating the channel parameters from the pilot channel. Increasing the power allocated to the power channel to improve channel estimation is undesirable.
Channel estimation can also conceivably be based on the signals of one or more of the higher-power data channels, for example the fundamental channel. The complexity of such an arrangement has made it undesirable or impractical in a CDMA system, and it may provide slow convergence, or no convergence, due to poor estimates of information symbols.
Accordingly, there is a need to provide improved channel estimation in CDMA systems.